In a multiple-access satellite system, a satellite carrying a single repeater provides multiple simultaneous transmission links between several Earth stations. A time-division multiple-access (TDMA) communication system is a multiple-access system wherein the time available is divided up into frames comprising a number of time slots one of which is allocated to each station for the purpose of transmitting a burst of data to another station. A typical Earth station includes a data terminal, a modem, and RF transmitting and receiving equipment. The modem must permit achieving the most efficient use of the passband of the repeater carried by the satellite, and the modulation technique generally used is the so-called phase-shift keying (PSK) modulation in conjunction with a coherent demodulation.
The PSK modulation technique is widely used and a detailed description thereof may be found, for example, in the book entitled "Data Transmission," by R. W. Bennett and J. R. Davey, Chapter 10, McGraw-Hill, New York, 1965, and in "Principles of Data Communication," by R. W. Lucky, J. Salz and E. J. Weldon, Jr., Chapter 3, McGraw-Hill, New York, 1968. Briefly, the PSK modulation technique requires that the sequence of bits to be transmitted be first converted into a sequence of symbols or pairs of symbols, wherein each symbol can take on a discrete number of values. These symbols or pairs of symbols are then transmitted one at a time, at instants which have a T-second spacing and are called signaling instants, in the form of a carrier phase change relative to the phase of the immediately preceding symbol. The coherent demodulation of a PSK signal is also a commonly used technique, a detailed description of which is provided, for example, in the two books referred to above. Briefly, the coherent demodulation technique involves measuring the phase of the receiving signal by means of a comparison with the phase of a local reference carrier. Obviously, a correct demodulation of the PSK signal requires that the reference carrier at the demodulator be synchronized with the transmitted carrier; it is therefore essential that some carrier synchronization means be provided in a coherent PSK demodulator.
When used in a TDMA communication system, a coherent PSK demodulator must include a carrier synchronization system that is very efficient in the presence of noise and very fast. The reasons for the latter requirement are that such a synchronization is necessary for each individual burst of data, taking into account the lack of carrier phase coherence between adjacent bursts, and that both the guard time interval between two adjacent bursts and the length of the carrier synchronization preamble at the beginning of each burst must be minimized to provide an efficient transmission. The reduction of the noise effects that is made possible by the carrier synchronization system is related to the fact that it is not feasible to provide RF modems intended to be used in TDMA systems with sophisticated noise-reducing systems similar to those used in modems designed for telephone channels.
There are two main types of carrier synchronization systems, namely, the phase-locked-loop systems and the passive-filter systems. The phase-locked-loop systems are discussed in many books and articles, and reference may be made, for example, to an article entitled, "Carrier Synchronization and Detection of Polyphase Signals," by W. C. Lindsey and M. K. Simon, published in IEEE Transactions on Communications, June 1972, pages 441-454, and to an article entitled, "Carrier Synchronization and Detection of QASK Signal Sets," by M. K. Simon and J. G. Smith, in IEEE Transactions on Communications, Vol. COM-22, No. 2, February 1974, pages 98-106. Systems such as those described in these articles are very effective as far as reducing the effects of noise is concerned, but do not permit achieving a very fast synchronization. Accordingly, preference was given to the use of passive filter systems of the type described, for example, in articles D2 to D6 of the Proceedings of the "Colloque International sur les Telecommunications Numeriques par Satellite," (International Conference on Digital Satellite Communications), Paris, Nov. 28-30, 1972. However, to increase the effectiveness of the passive-filter systems, it proved necessary to use the so-called quenching mode described, for example, in an article entitled, "A Fast Acquisition Coherent PSK Demodulator Incorporating Passive Narrowband Filters Operable in a Quenching Mode," by D. W. Baker and R. F. Robinson, in Satellite Communication Systems Technology, London, April 1975, pages 201-206 (an IEE publication), thereby increasing the complexity of such systems. It was found more practical, therefore, to concentrate on the phase-locked-loop systems and carry out additional development work to improve their performance characteristics.